During the development of in vivo amyloid imaging agents, an effort was made to use micro-positron emission tomography (PET) imaging in the presenilin-1 (PS1)/amyloid precursor protein (APP) transgenic mouse model of CNS amyloid deposition to screen new compounds and further study Pittsburgh Compound-B (PIB), a PET tracer that has been shown to be retained well in amyloid-containing areas of Alzheimer's disease (AD) brain. Unexpectedly, we saw no significant retention of PIB in this model even at 12 months of age when amyloid deposition in the PS1/APP mouse typically exceeds that seen in AD. This study describes a series of ex vivo and postmortem in vitro studies designed to explain this low retention. Ex vivo brain pharmacokinetic studies confirmed the low in vivo PIB retention observed in micro-PET experiments. In vitro binding studies showed that PS1/APP brain tissue contained less than one high-affinity (K d ϭ 1-2 nM) PIB binding site per 1000 molecules of amyloid-␤ (A␤), whereas AD brain contained Ͼ500 PIB binding sites per 1000 molecules of A␤. Synthetic A␤ closely resembled PS1/APP brain in having less than one high-affinity PIB binding site per 1000 molecules of A␤, although the characteristics of the few high-affinity PIB binding sites found on synthetic A␤ were very similar to those found in AD brain. We hypothesize that differences in the time course of deposition or tissue factors present during deposition lead to differences in secondary structure between A␤ deposited in AD brain and either synthetic A␤ or A␤ deposited in PS1/APP brain.

animals, supports the conclusion that the ABCA1 deficiency increases amyloid deposition. These results suggest that ABCA1 plays a role in the pathogenesis of parenchymal and cerebrovascular amyloid pathology and thus may be considered a therapeutic target in AD.The deposition of A␤ 3 in the brain parenchyma and vessels is a pathological hallmark of AD, and it is believed that A␤ plays a central role in the pathogenesis of the neuronal dysfunction and cognitive impairment during the progression of the disease. Unlike familial early onset forms of AD, which are caused by mutations in APP or presenilins, the cause of late onset AD (LOAD) remains unknown. The inheritance of the apoE ⑀4 allele (apoE4) is considered a strong and independent risk factor of AD and is associated with increased neuritic plaques and CAA (1-3). It has been reported that in APP transgenic mice the disruption of the apoE gene causes a dramatic reduction of parenchymal amyloid plaques and CAA (4 -6).Two independent groups have reported recently that the lack of ABCA1 causes Ͼ75% reduction of apoE protein level in the brain of ABCA1 Ϫ/Ϫ mice (7, 8). The decreased apoE level in the central nervous system was not related to apoE gene expression but likely was caused by increased metabolism of abnormally lipidated apoE containing lipoprotein (8). ABCA1 is a major regulator of cholesterol efflux, HDL metabolism, and reverse cholesterol transport (9, 10). Mutations in the ABCA1 gene cause severe HDL deficiencies, the most prominent of which is Tangier disease, which is characterized by the virtual absence of apoA-I and HDL, the accumulation of cholesterol in cells, and the prevalence of atherosclerosis (11-13). The transcriptional activation of ABCA1, and other genes involved in cholesterol metabolism, is controlled by nuclear liver X receptors ␣ and ␤ LXR␣/␤ (14, 15). Target genes of LXR␣/␤ have already been implicated in the control of APP proteolytic processing (16 -18). The effect was attributed primarily but not only to ABCA1. It was also reported that a set of genetic variants of ABCA1 modifies the risk for AD (19,20) in Scottish, Swedish, and English populations, although another study of American and UK populations found that ABCA1 variants do not appear to influence the risk of LOAD (21). Experiments with any of those genetic variants or complex AD model systems (APP transgenic/ABCA1 knock-out animals for example) have not been reported, and so there is no definitive conclusion about the role of ABCA1 in AD. To determine further the effect of ABCA1 on amyloid deposition, we used APP23 transgenic mice in which APPsw is expressed only in the brain. These mice develop compact amyloid plaques in brain parenchyma and CAA, reminiscent of the pathological features of AD (22)(23)(24). In this study APP23 mice were bred to ABCA1 knock-out mice, and A␤-related pathology was evaluated in APP23 mice with intact (APP23/ABCA1 ϩ/ϩ ) or disrupted (APP23/ABCA1 Ϫ/Ϫ ) ABCA1 gene. Our results demonstrate that in APP23 mice the lack of ABCA1 increases A␤ deposi...

Recent studies indicate that oxysterols, which are ligands for the nuclear hormone liver X receptors (LXR), decrease amyloid ␤ (A␤) secretion in vitro. The effect was attributed primarily to the ATP-binding cassette transporter A1 (ABCA1) transcriptionally up-regulated by ligand-activated LXRs. We now examined the effect of the synthetic LXR ligand T0901317, which can be used in vivo, on A␤ production in vitro and in APP23 transgenic mice. T0901317 applied to a variety of in vitro models, including immortalized fibroblasts from Tangier patients, and primary embryonic mouse neurons caused a concentrationdependent decrease in A␤ secretion, and this effect was increased by the addition of apolipoprotein A-I. The inhibition of A␤ production by T0901317 was cell-type specific, being more prominent in primary neurons than in non-neuronal cells. Tangier fibroblasts lacking a functional ABCA1 secreted more A␤ than control fibroblasts, thus demonstrating the role of ABCA1 in amyloid precursor protein (APP) processing and A␤ generation. T0901317 treatment of 11-week-old APP23 mice for 6 days showed a significant increase in ABCA1 expression and a decrease in the ratio of soluble APP (sAPP)␤-to sAPP␣-cleavage products. Most importantly, the treatment caused a statistically significant reduction in the levels of soluble A␤ 40 and A␤ 42 in the brain of these mice. Our experiments demonstrate that T0901317 decreases amyloidogenic processing of APP in vitro and in vivo, thus supporting the search for potent and specific LXR ligands with properties allowing therapeutic application.

The ATP-binding cassette transporter A1 (ABCA1) is a major regulator of peripheral cholesterol efflux and plasma high density lipoprotein metabolism. In adult rat brain we found high expression of ABCA1 in neurons in the hypothalamus, thalamus, amygdala, cholinergic basal forebrain, and hippocampus. Large neurons of the cholinergic nucleus basalis together with CA1 and CA3 pyramidal neurons were among the most abundantly immunolabeled neurons. Glia cells were largely negative. Because cholesterol homeostasis may have an essential role in central nervous system function and neurodegeneration, we examined ABCA1 expression and function in different brain cell types using cultures of primary neurons, astrocytes, and microglia isolated from embryonic rat brain. The basal ABCA1 mRNA and protein levels detected in these cell types were increased markedly after exposure to oxysterols and 9-cis-retinoic acid, which are ligands for the nuclear hormone liver X receptors and retinoic X receptors, respectively. Functionally, the increased ABCA1 expression caused by these ligands was followed by elevated apoA-I-and apoE-specific cholesterol efflux in neurons and glia. In non-neuronal and neuronal cells overexpressing a human Swedish variant of amyloid precursor protein, 22R-hydroxycholesterol and 9-cis-retinoic acid induced ABCA1 expression and increased apoA-I-mediated cholesterol efflux consequently decreasing cellular cholesterol content. More importantly, we demonstrated that these ligands alone or in combination with apoA-I caused a substantial reduction in the stability of amyloid precursor protein C-terminal fragments and decreased amyloid ␤ production. These effects of 22R-hydroxycholesterol may provide a novel strategy to decrease amyloid ␤ secretion and consequently reduce the amyloid burden in the brain.

Cramer et al. (Reports, 23 March 2012: 1503-1506) (1) demonstrated that treatment of APP/PS1ΔE9 mice with bexarotene decreased Aβ pathology and ameliorated memory deficits. We confirm the reversal of memory deficits in APP/PS1ΔE9 mice expressing human APOE3 or APOE4 to the levels of their non-transgenic controls and the significant decrease interstitial fluid Aβ, but not the effects on amyloid deposition.